In the early period of commercialization of television, the use of metal cones made large diameter cathode ray tubes (CRT's) practical. The first such tube, type 16AP4, was announced in 1948. Its development is described in the article by H. P. Steier et al., "Development of a Large Metal Kinescope for Television", RCA Review, March 1949, reprinted in Television, Vol VI (1950). In such tubes, a truncated metal cone formed the major section of the envelope. A glass faceplate was attached by a high temperature, glass-to-metal seal to a lip at the large end of the cone and a bell shaped glass similarly attached to the smaller end of the metal cone.
Improvements in mass production techniques resulted in large all glass CRT envelopes being produced at lower prices than metal cone types. Consequently, the use of metal cone CRT's as picture tubes in the television industry was discontinued. The use of specially designed metal cone CRT's continued on a small scale for use in radar displays. In these display tubes it is desireable that the faceplate be essentially flat in order to allow the use of mechanical markers on or directly above the surface of the horizontally mounted glass faceplate to provide radar operators with the means to indicate the location of radar targets. It was found that metal cone bulb technology of the traditional type was suitable for production of large envelope CRT's with essentially flat faceplates, without the major tooling and design problems associated with all glass, substantially flat faceplate envelopes.
Until my invention, the selection criteria for the metal alloy used for a metal cone CRT envelope has been determined by properties deemed desireable for a direct, high temperature glass-to-metal seal between the metal cone and glass faceplate. Chrome-bearing alloys have been the material of choice for such metal CRT cones for the past thirty years, usually of the type S.A.E. 446 or 430, although these alloys are relatively difficult to form into the desired shape. The faceplate glass used with these alloys for large diameter cones was a special glass selected to have an expansion coefficient of approximately 95.times.10.sup.-7 per .degree.C. in order to match the metal's properties.
In the early days of picture tube development, many types of sheet and plate glasses were available with various properties. Consequently, one could generally select a glass for a particular sealing design without difficulty. In recent years, however, flat glass is now produced on a worldwide scale almost entirely by the "Float" process developed by Pilkington Brothers Limited in the UK. Since the Float process is superior to other methods of producing flat glass, very few special flat glasses are now available at reasonable costs. Consequently, the special glass needed for the traditional flame sealing to metal cone alloys has become increasingly expensive and is available from only one source. The characteristics of Float glass are not very suitable for direct glass-to-metal sealing.
The "sealing" of a glass faceplate to a metal cone according to the prior art high temperature glass-to-metal seal method depends upon the ability of glass in the molten state to partially dissolve strongly adherent metallic oxides, thus forming a mechanically strong bond directly between the glass and metal. That sealing process, therefore, consists of oxidizing the metal, then melting the glass in contact with the oxidized metal and keeping the glass in the molten state until the bond is formed. The face plate and cone were placed in proximity to each other on a sealing machine, the face and cone were rotated and heated uniformly until the glass temperature was close to the annealing point (the temperature at which glass is fluid enough to allow stress relief without deforming). At that time, the sealing heat was applied at the edge of the faceplate and a lip of the cone, so that the faceplate glass in contact with metal was melted at a temperature of approximately 1000.degree. C. and the seal was formed. Air pressure was controllably applied inside the cone during this operation to hold the faceplate in position and to work and form the seal. The shape of the seal was important, because a smooth contour eliminated points of high stress concentration in the seal area which might weaken the glass and cause glass breakage. At the completion of the sealing operation, the envelope was transferred to an oven maintained near the annealing temperature of the glass and allowed to temperature-equalize.
It is well known in the art that properly designed complex automatic or semi-automatic flame-sealing equipment can provide a fast and efficient means of producing high temperature glass-to-metal sealed metal and CRT envelopes. A most important factor in the flame sealing technique is "fire setting" and "running-in" of the sealing process; however, the fire setting and running-in steps invariably involve the initial production of rejects of scrap until successive small adjustments are made to provide the necessary quality and repeatability. Consequently, in the case of large CRT envelope assemblies, the flame-sealing technique is primarily suited to continuous production of large quantities. Since the current use of metal cone CRT's is limited to special radar displays and similar non-mass-produced products produced in small runs, the traditional flame sealing method of producing metal cone envelopes is extremely costly and inefficient.
The sealing of two glass surfaces, such as the edge of a glass CRT envelope body and a glass face panel, by use of a solder glass is well known. Such materials are either low melting point glasses or "frits", glass materials which change from a glassy state to a crystalline or ceramic state upon application of heat. Glass-to-glass seals using solder glasses are commonly employed to join glass faceplates to glass cones in the manufacture of shadow mask color CRT's. Such sealing materials have also been used for sealing relatively thick, small diameter glass or glass fiber-optic faceplates to metal envelopes of image intensifier tubes. See, for example, U.S. Pat. No. 3,916,240. In such tubes, the major sealing surface of the faceplate is parallel to the major plane of the faceplate and the faceplate thickness at the seal area is approximately 1/16 inch or more per inch of opening spanned by the faceplate.
To the best of my knowledge, however, solder glass seals have not been successfully used and have not been seriously considered suitable for sealing large, relatively thin glass faceplates to metal CRT cones. I have found that such seals can in fact be made in accordance with my invention, avoiding the high sealing temperatures, the critical control of the seal contour and the possible distortion of the faceplate which occurred in the prior, high temperature glass-to-metal sealing method. In accordance with my invention, it is unnecessary to use difficult to form chrome-bearing alloys and special glasses, and special treatment to enhance the oxides of the sealing area is unnecessary.
If one merely substitutes a solder glass seal at the interface of the large glass faceplate and metal cone, following a similar metal lip contour to that employed for flame sealed metal cone envelopes, the mechanical properties of the envelope are quite unsatisfactory. Such an envelope will not withstand a 35 pound per square inch absolute test, the minimum required to assure the safe processing and use of the CRT envelope. Thus, in accordance with the present invention, the shape of the interface between glass faceplate and metal cone differs from that of the earlier tubes mentioned above.
Further details concerning the objects and advantages of my method and apparatus will be clear from the drawings and the following detailed description.